1. Field of the Invention
The present invention relates to a process for catalytically removing nitrogen oxides from an oxygen rich exhaust gas containing nitrogen oxides and hydrocarbons, exhausted from, for example, boilers, automobile engines. More specifically, it relates to a process for removing nitrogen oxides using a catalyst extremely superior in activity and durability.
The term "oxygen rich exhaust gas" used herein is intended to mean an exhaust gas containing oxygen in an amount exceeding the amount of oxygen necessary for completely oxidizing reducing agent such as carbon monoxide, hydrocarbons contained in the exhaust gas. Specific examples of such an exhaust gas include exhaust gases discharged, for example, from internal combustion engines of automobiles operating at a high air/fuel ratio (i.e., in the lean burn region).
2. Description of the Related Art
The processes for removing nitrogen oxides from an exhaust gas discharged from boilers, automobile engines, and the like, which are in actual use are the process of selective catalytic reduction using ammonia in the presence of a catalyst or the process of nonselective catalytic reduction which passes the exhaust gas through a catalyst and reduces it by the unburnt carbon monoxide and hydrocarbons.
Japanese Unexamined Patent Publication (Kokai) No. 60-125250 proposes a copper-ion-exchanged zeolite as a catalyst for the catalytical direct decomposition of nitrogen oxides in the absence of a reducing agent.
For purifying the exhaust gas of diesel engines or lean burn engines designed for reduced fuel consumption, further, there has been proposed a catalyst which includes a base (or non-noble) metal in the zeolite etc. as a catalyst capable of selectively reducing the nitrogen oxides by a reducing agent of the unburnt carbon monoxide, hydrocarbons, etc. even in an oxygen rich atmosphere (Japanese Unexamined Patent Publication (Kokai) No. 63-100919).
These proposed catalysts, however, suffer from problems, in particular their durability, at a high temperatures, and therefore, are not yet suitable for practical use.